This invention relates generally to electrocardiogram (ECG) recording and display equipment and more particularly to a method for graphically annotating ECG data.
FIG. 1 is a graph showing the printout of three channels from an ECG. Each ECG signal from channels 1, 2, and 3 consists of a sequence of ECG cycles 11 each representing an individual heart beat. Each ECG cycle 11 may include three separate wave complexes defined as P, QRS, and T. Each wave complex defines a specific stage in the patient's heart beat. The three waves' complexes are shown bracketed within the first ECG cycle 11 of ECG channel 1.
The ECG signals in channels 1, 2, and 3 are derived from electrode readings. An "electrode" is an adhesive patch (or metal plate) that is placed on the patient's skin and connected to a wire coming from the cardiograph. For a routine 12-lead resting ECG, ten electrodes are required. An electrode is attached to each of the limbs (right/left arm/leg: RA, LA, RL, LL) and six electrodes are arrayed across the left chest at anatomically-determined positions (V1 through V6).
From the signals captured by the electrodes, two sets of ECG "leads" are generated. The "limb leads" are formed from the RA, LA, and LL electrodes as follows:
I=LA-RA PA1 II=LL-RA PA1 III=LL-LA PA1 aVR=RA-0.5*(LA+LL) PA1 aVL=LA-0.5*(RA+LL) PA1 aVF=LL-0.5*(LA+RA)
Similarly, the "chest leads" are formed by subtracting the average of RA+LA+LL from the corresponding chest electrode at each sample point. For example, lead V1=electrode V1-[(RA+LA+LL)/3].
The channels 1, 2, and 3 refer to a standard 3.times.4 presentation of ECG waveforms on the ECG report. Channel 1 contains 2.5 seconds of ECG leads I, aVR, V1, and V4. Similarly, channel 2 contains leads II, aVL, V2, and V5 and channel 3 contains leads III, aVF, V3, and V6. FIG. 1 shows the first 2.5 seconds of leads I, II and III. The additional leads are shown in their entirety in FIG. 5.
In a given heartbeat, the ECG signal read by each electrode is different. The electrodes are placed in different locations. ECG electrodes are "directional" in their ability to detect electrical heart activity. Each electrode records that portion of the net heart electrical activity that moves towards or away from that electrode's position. However, net electrical activity of the heart that moves perpendicular to the electrode's directional view is not recorded. Therefore, each electrode has a different perspective on heart activity. The directional variance in the different ECG electrodes creates differences in the magnitude, direction, and relative position of each ECG wave. Thus, electrodes placed in different locations on a patient, such as, on the limbs and chest, produce varied responses to the same heart beat.
Each P, QRS, and T wave has a specific beginning referred to as a wave onset and an end referred to as a wave offset. Because of the different perspectives of the electrodes, there is a built-in and normal variation from lead to lead of the onset and offset for each P, QRS, and T wave. In fact, in some leads in some ECGs, a P or QRS or T may not be "seen" at all when the net electrical activity in the heart takes place perpendicular to the electrode's viewpoint.
Additional variances in ECG readings are also caused by noise, for example, from body movements or from AC power interference. The amount of shift in the onset and offset of ECG waves also depends upon how the ECG system processes the noise. For example, there may be smaller shifts between recorded ECG signals if a detection algorithm is capable of detecting and subsequently rejecting noise.
In the presence of noise in the ECG and as a result of imperfect algorithms for determining true onsets and offsets, the variation in onsets/offsets between leads also contains a component that is due to "measurement error". Sometimes these are true errors when the measurement program becomes confused. More frequently these errors are better described as simple inaccuracies in determining the onset/offset times.
Typical ECG systems such as the Hewlett-Packard PageWriter XLi cardiograph, detect the P, QRS, and T waves and then produce numerical results. The measurements shown at 10 are only three of more than 800 numerical results, collectively known as the "measurement matrix", produced by the measurement program for an ECG. There are more numerical results in a "measurement table" from which the onsets and offsets for the annotation process are extracted. The numerical results such as 10 are derived according to the onset and offset of each P, QRS, and T wave. Therefore, if the onset or offset of a wave complex is inaccurate, the numerical data may also be inaccurate.
To verify that the numerical results 10 are correct, a physician examines the ECG signals in channels 1, 2, and 3. If any anomalous results appear in the individual ECG signals, there is a probability that the numerical results may be inaccurate. Thus, the physician is either notified that the numerical data is potentially wrong or is reassured that the numerical data is reliable. The physician examining the ECG signals, however, typically does not know where the ECG system designates the specific onset and offset locations for each P, QRS, and T wave. Therefore, it is difficult to accurately assess whether the ECG signals detected by the measurement program in channels 1, 2, and 3 are accurately aligned or are erroneously offset to such a degree where the validity of the numerical data 10 is in question. Thus, there is no quick and accurate way shown in FIG. 1 to correlate the measurement program's decisions regarding onsets and offsets of wave complex components to verify numerical ECG data.
One method for verifying the numerical ECG data 10 is shown in FIG. 2. Referring to FIG. 2, tick marks are superimposed at the onset and offset of each P, QRS, and T wave. For example, a tick mark 12 and a tick mark 14 are superimposed at the derived onset and offset points, respectively, on the first P-wave of the ECG signal in lead I. Correspondingly, a tick mark 16 and a tick mark 18 are superimposed at the derived onset and offset points, respectively, on the first P wave of the ECG signal in lead II and tick marks 20 and 22 are superimposed on the first P wave of the ECG signal in lead III.
Each tick mark defines either the beginning or end of the associated P wave. Thus, an ECG system operator can verify that the wave onset and wave offset for one particular wave complex are aligned with the onset and offset for the same wave complex in the two adjacent channels. For example, the ECG system operator looks at the relative horizontal positions of tick marks 12, 16, and 20. If each tick mark is substantially aligned in a vertical column, it is likely that each P wave was accurately measured and detected at approximately the same time. It is then more likely that the numerical data 10 is based on accurate ECG measurements. The onset and offsets of the QRS wave and the T wave in ECG lead I are similarly identified with tick marks.
Superimposing tick marks on the ECG signals as shown in FIG. 2, however, has two distinct disadvantages. The first disadvantage is that the tick marks are obstructive to the ECG waveform data at the onset and offset locations. Specifically, the tick marks hide the ECG signal and disrupt normal view of each channel. The second disadvantage is that it is difficult to correlate the onset and offset locations for the same wave complexes on different ECG channels. For example, slight shifts in time between different waveforms is difficult to discern because of the spacial displacement between adjacent waveforms. Specifically, there is too much distance between tick marks 12, 16 and 20 to accurately determine the amount of relative shift between the derived onset of each P wave.
Accordingly, a need remains for displaying ECG data so that information between associated ECG signals can be quickly and accurately correlated.